Internal combustion engines are utilized extensively to power vehicles and equipment and, as a result, their composition and function will be familiar to one skilled in the art. A cylinder of a typical internal combustion engine undergoes four different sequential strokes during a single complete cycle of the engine: an intake stroke, a compression stroke, a combustion (or “power”) stroke, and an exhaust stroke. FIG. 1 shows a sectional view of a cylinder 100 during its compression stroke. The cylinder can be broadly divided into a bottom end 105 and a top end 110, which are separated by a head gasket 115. The bottom end comprises a piston 120 which is connected to a crankshaft 125 by a connecting rod 130. The crankshaft sits in a crankcase 135. The piston includes piston rings 140 that act to seal the region above the piston from the crankcase below the piston. The top end, in turn, comprises a spark plug 145, an intake manifold 150, and an exhaust manifold 155. An intake valve 160 and an exhaust valve 165 are driven by camshafts 170. Both the bottom end and the top end of the cylinder are cooled by various liquid cooling ducts 175 that allow water to be circulated through the cylinder. Additional information on these elements and their function within an internal combustion engine may be found in, for example, R. Stone, “Introduction to Internal Combustion Engines,” Third Edition, SAE International, November 1999, which is hereby incorporated by reference herein.
As indicated in FIG. 1, both the intake valve 160 and the exhaust valve 165 are closed when the cylinder 100 is in its compression stroke, allowing the combustible gases residing above the piston 120 to be compressed in preparation for ignition by the spark plug 145. In an internal combustion engine, the “compression” is the maximum pressure of the gases occupying the volume above the piston when the piston is in its compression stroke. Many gasoline automobile engines are specified by their manufacturers to have a compression of about 120-200 pounds per square inch (psi), although higher values are also sometimes utilized. A compression within the manufacturer's specification is desirable because it allows an internal combustion engine to efficiently extract mechanical energy from a given mass of air-fuel mixture. Unfortunately, several defects can occur in an internal combustion engine that can cause one or more of its cylinders to lose compression because of gas leakage from the volume above the piston. These defects include defects in the cylinder rings 140 that can cause excessive gas to leak into the crankcase 135 (i.e., excessive blow-by); defects in the intake valve and exhaust valve that can cause excessive gas to leak into the intake manifold 150 and the exhaust manifold 155, respectively; and defects in the head gasket 115 that can cause excessive gas to leak into the liquid cooling ducts 175, which are part of the engine's liquid cooling system.
The conventional manner of measuring compression involves removing a spark plug and using a pressure gauge to measure the pressure generated in a cylinder while the engine is being cranked by its starter motor. Unfortunately, such a methodology is work intensive and time consuming. As a result, other methods have been developed for measuring compression in internal combustion engines. Some alternative methods involve, for instance, measuring the voltage level or current draw on a battery as a function of time while that battery is powering a starter motor that is cranking the engine. Such methods are described in, for example, U.S. Pat. No. 5,585,717, entitled “Method for Measuring Starter Motor Current to Determine Engine Status,” to Eriksson et al.; and U.S. Pat. No. 5,663,493, entitled “Apparatus and Method for Measuring Relative Compression,” to Gerbert et al; which are not admitted as being prior art by their inclusion in this Background Section. These particular methodologies typically use peak heights in the voltage level or current draw waveform data to determine relative compression. Nevertheless, because the voltage level and current draw data gathered in this manner tend to include large fluctuations in baseline levels, these methods of analysis may be inaccurate and misleading. Moreover, while these techniques may, in some cases, be able to provide information about relative cylinder compression, they do not provide the user with any additional information about the root cause of any abnormal compression values.
For the foregoing reasons, there is a need for methods and apparatus that are operative to allow a user to conveniently and accurately determine the relative compression of each of the cylinders in an internal combustion engine, while, at the same time, also providing the user with useful additional information about the root cause of any abnormal compression values.